To improve the overall efficiency of a gas turbine engine, a heat exchanger or recuperator can be used to provide heated air for the turbine intake. The heat exchanger operates to transfer heat from the hot exhaust of the turbine engine to the compressed air being drawn into the turbine. As such, the turbine saves fuel it would otherwise expend raising the temperature of the intake air to the combustion temperature.
The heat of the exhaust is transferred by ducting the hot exhaust gases past the cooler intake air. Typically, the exhaust gas and the intake air ducting share multiple common walls, or other strictures, which allow the heat to transfer between the two gases (or fluids depending on the specific application). That is, as the exhaust gases pass through the ducts, they heat the common walls, which in turn heat the intake air passing on the other side of the walls. Generally, the greater the surface areas of the common walls, the more heat which will transfer between the exhaust and the intake air. Also, the more heat which transfers between the exhaust and the air, the greater the efficiency of the heat exchanger will be.
As shown in the cross-sectional view of FIG. 1, one example of this type of device is a heat exchanger 5, which uses a shell 10 to contain and direct the exhaust gases, and a core 20, placed within the shell 10, to contain and direct the intake air. As can be seen, the core 20 is constructed of a stack of thin plates 22 which alternatively channel the inlet air and the exhaust gases through the core 20. That is, the layers 24 of the core 20 alternate between channeling the inlet air and channeling the exhaust gases. In so doing, the ducting keeps the air and exhaust gases from mixing with one another. Generally, to maximize the total heat transfer surface area of the core 20, many closely spaced plates 22 are used to define a multitude of layers 24. Further, each plate 22 is very thin and made of a material with good mechanical heat conducting properties. Keeping the plates 22 thin assists in the heat transfer between the hot exhaust gases and the colder inlet air.
Typically, during construction of such a heat exchanger 5, the plates 22 are positioned on top of one another and then compressed to form a stack 26. Since the plates 22 are each separate elements, the compression of the plates 22 ensures that there are always positive compressive forces on the core 20, so that the plates 22 do not separate. The separation of one or more plates 22 can lead to a performance reduction or a failure by an outward buckling of the stack 26. As such, typically the heat exchanger 5 is constructed such that the stack 26 is under a compressive pre-load.
Applying a high pre-load reduces the potential for separation of the plates 22. However, this approach does have the significant drawback that all the components of the core 20 are placed under much greater stress than they would be without the pre-loading. In addition, the pre-loading requires that the structure supporting the stack 26 must be much stronger and thus thicker. This pre-load assembly or support structure 40 collectively includes strongbacks 28, tie rods 30, as well as the shell 10 structure. This support structure 40 adds to both the weight and the cost of the heat exchanger 5.
Because the support structure 40 supports the core 20 and is not a heat transfer medium, the components of the support structure 40 are typically made of much thicker materials than that of the core 20. Unfortunately, these thicker materials cause the support structure 40 to thermally expand at a much slower rate than the quick responding core 20, which has the thin plates 22. The thickness (and thus the thermal response) of the support structure 40 will also be affected by the amount of the pre-load it must apply to the core 20.
Differential thermal expansion between elements of the heat exchanger 5 will cause a compression load to be applied to the quicker expanding sections (e.g. the core 20 and specifically the stack 26). As noted, a compression load is also applied to the stack 26 by the application of a pre-load. Compressive forces from pre-loading and differential thermal expansion can cause a variety of problems, such as buckling, fatigue failures and creep. Buckling is particularly problematic as it results in the stack 26 expanding outward (laterally) in one or more directions. This outward expansion causes the plates 22 to separate from one another, resulting in a nearly complete destruction of the heat exchanger. Fatigue and creep frequently occur when heat exchangers are repeatedly cycled between hot and cold stages. Depending on the particular application, a turbine (not shown) attached to a heat exchanger can be started, ran for a short period of time and then shutdown, over and over. One example of such cyclic use is a turbine and heat exchanger apparatus employed in the production of electric power. Typically, such devices are run only during recurring periods of peak power demand.
An additional source of loading on the heat exchanger can be from the airflow in the core 20. When the inlet air in the core 20 is pressurized, the core 20 will want to expand out against the support structure 40. This increases the amount of support structure needed to contain the core 20, which further reduces the thermal response of the supporting structure 40.
Prior approaches to providing for differential expansion between the core 20 and the shell 10, have included providing a gap or space for the core to expand into. However, the use of such a gap greatly reduces the efficiency of the heat exchanger by allowing much of the exhaust gas to pass around the core and not through it. Because of the gas pressures typically involved, even a very small gap can allow a great deal of exhaust gas to bypass the core. When the exhaust gas bypasses the core, less heat transfers to the intake air, and as a result, the overall efficiency of the heat exchanger (and thus of the turbine) drops dramatically.
Therefore, a need exists for a heat exchanger which allows for differential thermal expansion between the core and the supporting structure, thereby preventing core buckling, fatigue failures, creep or other similar problems. The heat exchanger must however apply, throughout the differential expansion, a force (e.g. pre-load) to the core, which is sufficient to keep the core plates from separating or otherwise deviating from their positions. In addition, the heat exchanger must maintain a seal between the core and the shell, so to prevent the gases from bypassing the core, which would otherwise reduce the efficiency of the heat exchanger. Further, such an apparatus should be relatively simple in construction and operation to minimize its cost, weight and complexity.